Circular (Iowa State College. Agricultural
Experiment Station)
Iowa Agricultural and Home Economics
Experiment Station Publications
4-1913
Bacteria and Soil Fertility
P. E. Brown
Iowa State College
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Recommended Citation
Brown, P. E., "Bacteria and Soil Fertility" (1913). Circular (Iowa State College. Agricultural Experiment Station). Paper 7.
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Circular No. 7
April, 1913
BACTERIA AND SOIL
FERTILITY
AGRICULTURAL EXPERll\IENT STATION
IOWA STl.TE COLLEGE OF AGRICULTURE
AND THE MECHANIC ARTS
Agronomy Section
Soils
Ames
Iowa
OFFICERS AND STAFF
IOWA AGRICULTURAL EXPERIMENT STATION
STATE BOARD OF EDUCATION.
Hon. J. II. Trcwln, Cedar 1\aplds.
Hon. A. B. Funk, Spirit Lake.
Ron. Ueorge T. Baker, Davenport.
Hon. Charles II. Brenton, Dallas Center.
Hon. E. P. Schoentgen, Council Blurrs.
Hon. Parker K. Holbrook, Onawa.
Hon. D. D. ~Iurphy, Elkader.
Hon Roger Leavitt, Cedar Falls.
non. Henry M. Eicher, washington.
OFFICERS.
Hon. J. H. Trewln, Cedar Rapids . . • . . . . . . . . • . . . • • • • • • • . • • • • • • • President
Hon. D. A. Emery, Oltumwa .....•............•....•.•.••••... Secretary
FINANCE COMMITTEE.
Hon. W. R. Boyd, President, Cedar Rapids.
Hon. Thos. Lambert, Sabula.
Hon. D. A. Emery, Secretary, Ottumwa.
AGntCt:LTURAL EXI'ERDIE:'iT STATION STAFF.
Raymond A. Pearson, ~1. S. A., LL. D., President.
c. F. curtiss. ~1. s. A.• D. S., Director.
J. B. Da\'ldson, B. S., ~1. S. A., Chler in Agricultural Engineering
\V. H. Ste\·cnson, A. B., B. s. A., Chler In Agronomy.
H. D. Hughes, ll. s., Chler In Farm Crops.
L. C. Burnell, ~1. s. A., Assistant Chler In Cereal Breeding.
·
P. E. Brown, B. S., A. ~I., Pit. D.. A5slstant Chler In Soli Bacteriology.
John Buchanan, B. s., Superintendent or Co-operative Experiments.
Charles R. Fort•sl, Field Snperlntl'ndent.
E. H. Kellogg, B. S., Assistant In Soli Chemistry.
Robt. Snyder. B. S., M<slstant In Soli Chemistry.
\Y. 11. l'ew, B. S. A., Chler In Animal Husbandry.
11. fl. Klldec. B. s. A., Assistant Chic£ In Dairy Husbandry.
J. M. En·ard, ~1. S., Assistant Chler In Animal llusllandry.
Geo. ~1. Turpin, B. s., Assistant Chler In Poultry llusbandry.
D. B. Adams, llerdsman.
R. E. Buchanan, ~1. S., Ph. D., Chlct In Bacteriology; Associate In Dairy ;mel
Soli Bacteriology.
I.. H. l'nmmcl, B.•\gr., ll. s., Ph. D., Chlet In Bolany.
Charlolle ll. King, A~~lstant Chter In Botany.
Harriette Kellogg, Assistant In Botan)·.
A. \V. Dox, B. S., A. ~I., Ph. D., Chler In Chemistry.
R. E. Neidig, ll. S. Assistant In Chemistry.
W. G. Gae<sler, B. s., Assistant In Chemlstr}·.
S. C. Guernsey, ~1. s .. Assistant In Chemistry.
Eugene Ruth. ~1. s., :\~slstant In Chemlstrr.
J. c. Reese, B. s.~. Asslst:mt In Chemistry.
M. Mortensen, B. :;, A., Chter In Dairying.
B. "'· Hammer, 8. S. ,\ .. A;slstant Chlrr In Dair-r llarlerlnlnll'Y·
H. E. Summers, B. s., Clller In Entomolog~·.
R. L. \Yehster, A. B., Assistant Chic£ In Entomolo!l'~·.
S. A. Beach, B. s. :\., ll. S., Chlct In llortlculturr ami rnrestry.
Laurt>nz Greene, 8. s. A.. ~1. S. A. Assistant Chh•r In lhH·tlcnillllf'.
G. B. Mac Donald, B. S. F., ASSIStant Chlet Ill Fort'!'try.
J. H. Allison, Assistant In Plant Introduction.
T. J. ~laney, Assistant In Horticulture •
C. II. Stan!l'r. D. V. ~1.. ChiH In \'eterlnat·y ~lt·dl<·ilu~.
F. \\'. llf'f'kman. Ph. II .. Bnllt•:ln F.tlii'J~.
F. C. f:nillltrn. l'hflln!lt'apher.
BACTERIA IN RELATION TO SOIL
FERTILITY
BY P. E. BROWN.
The problem of obtaining maximum crops Is centuries old but the
agricultural world Is still awaiting Its solution.
It Is known now that the crotJ any soli will yield under
particular climatic conditions depends on the character and condition or that soli. If the soli Is poor and Infertile, the crop may be
expected to be rmall; If It Is rich and climatic conditions are fa\·orable, the yields should be large. The real problem, then, Is how to
make the soli fertile and how to keep It so. If It Is poor, then Improvement Is necessary; If It Is good, further Improvement may pay.
There are probably few soils so poor that Jlroper methods cannot put
them on a paying basis; on the other hand, there are no soils so rich
that they will always continue to be fertile.
Therefore, a study or soli fertility, or the crop )Jroduclng power
or a soli under gh·en climatic conditions Is or ,·an Importance. ~len
must ha\·e kno\\·ledge of the fertility of a soli if they would
properly regulate Its !lli)Jport lllllfer so that the needed sup11IY of
plant food may be amlllable for crojJ 11roductlon.
·
Plant food consists of those chemical elements which are essential for the growth of plants and Include~ a large number of sub·
stances. Among thel!e, nitrogen, potassium, tJbosphorus, and sulfur
are most likely to be lacking !n soils. In rare cases other elements
rna)' be deficient, but In normal soils, If enough of these four elements Is present In a !10hthl4'. lltnllnltlr form, tl:e support power of the
soil Is satisfactory.
~!any soils contain enough of these four necessltr~· elelllents but
they are locked up In an Insoluble and una,·allable form and hence
must be changed and made soluble to be of use to crops. How Is this
change accomtlllshed! What determines the production or soluble
plant food In the soli! What regulates the support JlO\\'er or the
soli? These are the questions which require a definite answer.
The factors which bring about the caange of Insoluble substances
Into soluble In the soil, may be grou; ed Into t!Jree cla11~es, )Jbysi('al,
chemical and bactedlologlcat.
The pb)·siclll factors, such as light, heat, water, air, etc., and t!Je
chemical factors, such as soli acidity or sourness, the character of the
chemical comaJOunds present, etc., have been known and studied for
many )'ears. The bacteriological factors ha\·e come Into prominence
only quite recently, but now they are recognized to be of as much, If
not more Importance than the other tl\·o ~rott)lS.
It ball been wrll 11ald that these thr('e ~routH! of factors working
4
together constitute the "commissary department for the army of
plant life."
"~hile, therefore, it is quite generally known now that the soU is
the home of myriads of microscopic plants called bacteria, and that
these have much to do with fertility, there is still a great deal of
haziness about the subject In the public mind.
This circular Is Intended to clear up this haziness and to present
In plain terms our present knowledge about bacteria in the soil and
the part they play In making plant food available.
FACTS ABOUT BACTERIA
Before considering the part which bacteria take in liberating
plant food It Is well to bear in mind the fundamental facts about bacteria.
Bacteria are minute plants, consisting of single cells.
These
cells are made up of a cell-wall, and the living substance or protoplasm within the cell-wall. In the protoplasm the life processes are
carried on. When food Is absorbed, it passes into the cell through the
ceiJ-wall and the necessary portions are taken up by the protoplasm
'while the waste is eliminated, passing O\lt through the cell-wall.
FIG. 1.-Types of Bacteria, Cocci, Bacilli and Spirilla from
many times magnified.
left to right,
Ty]JC!O. There are three main types of bacteria grouped according
to form. They are the cocci or spheres, the bacilli or rods, and the
~l•irflla or spirals.
These groups ha\·e bepn popularly described as
billiard balls, lead pencils and corkscrews. By far the largest number of soil bacteria belong to the group of bacilJI or rods.
)[nlti]Jlfcntil'n •.. These simple organisms multiply by fission, that
is, one cell divides Into two equal parts, which may separate or may
remain united. The spherical bacteria, due to their method of multiplication, frequently appear as packets of varying numbers of organIsms, while the rods and spirals appear singly or in chains.
~Rate of.Ineremie, The splitting of one organism Into two may be
completed In twenty minutes to one half hour under very favorable
conditions. At this rate, in one day one organism would become about
300,000.000,000,0GO. As a matter of fact, however, sueh a rapid multi-
5
FlO. 2-MoUie and non-moUie Bacteria, showing types of flagellation.
plication cannot occur, as food conditions do not remain satisfactory
and the growth of the organisms gh·e~ products that restrict development
Size.
Bacteria are very small, ranging from 1-iiO,oon to 1-1,000
of an Inch In length and averaging about 1-25,000 of an Inch.
lloUon. llany bacteria have fln~~:elln, or long thread-like appendages, by means of which they mo\·e about freely In any liquid
in which they may be growing. These flagella mar be attached In'
various ways, singly at the end of the organism, In tuft!l of several
at one or both ends, or scattered Indiscriminately o\·er the organism.
SJJorulntJon, Some bacteria under certain comlltlons, produce socalled SJIOres, or cells surrounded by a very resistant cell-wall. They
will remain alh e llractlcally Indefinitely If not expo~ed to extremes
of heat or cold and tr kept dry; and when 11laced under fa\·orable
con11ltlons will germinate and produce acth·e bacteria again.
Sterilization. Sterilization Is any )Jroce>s whereby bacteria are
killed. The methods or sterilization whch ma)' be Cl111Jiored, deJ1ending on the materials which are to be freed from bacteria, are dry beat,
steam under 11ressure, Intermittent sterilization, which consists in
bentlng at the tenlJlerature of bolllng water for one-half hour on three
successh·e days, chemical substances, sunlight, etc.
fnllnres. Knowledge about !lterlllzatlon, the food nee:lecl by
bacteria, and other necessities for their growth, Is n~e:l In the
laboratory by growing bacteria on cnltnre mt'dl~ that Is, an)· substance which will 1mpport their growth. Euch a growth !11 called a
cnltnr~ and when onlr one kind or org-anl5m !10 prPsent, sut'h a culture
becomes a pnrf' rnltnre.
II
'
FlO. a-spore produc:tlon, ahowlng method of produc:tlon br nrloua Bacteria.
6
FIG. 4.-A pure culture of Bacillus prodigious, an organism that produces red
pigment.
Classes. All bacteria may be Included In one of two large classes
depending on their functions, or the character of their activities. These
are the parasites and the SllltrO}thytes. The parasites Include all the
disease producers. The saprophytes are the decay producers. Many
people think of all bacteria as connected with disease. They know that
typhoid, diphtheria, tuberculosis and other diseases are caused by
bacteria and fall into the error of believing that all organisms are
active in causing some dread disease. Such is far from being the
case, however. The saprophytic, or decay bacteria, are invaluable.
They have been called the "link between the world of the living and
the dead." They transform dead materials back Into living matter and
thus complete the cycles through which, In nature, all substances must
go. Bacteria are everywhere, in the air, the water, the soil, but contrary to the popular belief, which is that we are becoming "bacteria
crazy," such general distribution Is no cause for alarm but rather a
source of benefit. A bacteria-free world would soon be a dead world.
BACTERIA IN THE SOIL
Enormous numbers of bacteria inhabit the soil, some of them
harmful, but the vast majority beneficial. Actual counts have shown
that the numbers present in soils may vary from a few thousand
per gram (about 1-30 of an ounce) to over fifty million per gram.
CONDITIONS AFFECTING THE GROWTH OF BACTERIA.
There are certain conditions affecting the growth of bacteria.
A discussion of these has purposely been omitted up to this point In
7
order to consider them from the standpoint of the soli. In other
words, the bacteria In the soli, just as In any other environment, are
greatly lnfuenced by certain physical and chemical conditions. These
conditions are moisture, temperature, aeration, reaction, and food
supply.
Moisture. A proper amount of water In the soil Is as necessary
for the growth of bacteria as Cor crops. Either excessive moisture
or severe drought Interferes with bacterial growth very considerably.
Many organisms are _killed by too much moisture, many others, by
Insufficient moisture. Merely drying out a soli by exposing It to the
alr, however, '\\"Ill not kill the bacteria presenl Such soils, kept Cor
years In an air-dry state, have been shown to contain certain bacteria
which had evidently been In a dormant condition. Such farm practices
as drainage, which removes excessive water, or cultivation, which
prevents undue loss of water by evaporation. have an Important Influence on the bacteria In the soil. Recent work has shown that
small variations In moisture are of little Influence on bacteria In the
field, other factors apparently being of greater Importance, but when
variations are large, then moisture becomes the governing factor.
Temperature. Every organism grows the best at a certain tem·
perature which Is called Its OJitlmnm lemJ•erntnre. Each also baa
so-called mnxlmnm and mfnfmnm temperatures at which points growth
ceases. The optimum temperature for most soli organisms ranges
from 65°-90° F., although of course there are exceptions to this statement :\lost organisms are not killed by excessive cold but merely
remain dormant In fact, It has been shown recently that certain
FlO. 6-Nodultl on Alfalfa root..
8
bacteria are alive and active . in soil in late winter, at a soil te~~
perature somewhat below the freezing point. It Is. generally considered, however, and with reason, that the greatest bacterial activities
occur in the soil during the summer and are then of the m<?st ·lllg~
nlficance. Those occurring during the winter are of Importance only
when fall applications of manure are made.
·
Aeration. Depending upon their requirements as to air, bacteria
may be divided Into three classes: the aerobes, which require air
for their growth; the anaerobes, which grow only In the absence of
air, and iacultnthe anaerobes, which prefer air·, 'but will grow without
it. In general It may be said that the beneficial bacteria In the soil
need air. Hence In heavy clay soils where there· Is not enough air,
methods which Increase the circulation of oxygen In the soil, increase
bacterial activities; these Increase the solution of plant food and this
ultimately lnc.reases crop production. On ·the other hand, If there
Is too much air present, as In light sandy soils, the bacterial activIties will be too great and the humus will be burned up too rapidly,
plant food will be produced In too large quantities to be utilized by
the crops, and more or less extensh·e losses of valuable soil elements
will occur. :\Iethods must be practiced with such soil, which will
make It more comJlact and prevent the exc.:!sslve circulation of air,
reducing bacterial activities to what Is best.
Heactlon. The reaction of a soil Is Its relative acidity, or "sourness" or alkalinity. The reaction means much from the bacterial
standpoint. Soils which have become acid or sour are notably unproductive and this is largely due to the fact that the growth of
beneficial bacteria in such soils is checked. Eome bacteria are probably fa\·ored by add conditions but those organisms which bring about
the solution of the illlJJOrtant plant food constituents refu~e to de\·elop
In aeld soils. Such an unfavorable ·condition may be remedied by
application of g-round limestone or caustic lime In mrying amounts.
Applicati~ns of ground limestone to sour soils have been shown to
be followed by increased beneficial bacterial acth·ities and later by
Increased crop production. (Further Information regarding application of lime to Iowa soils may be found in" Circular Xo. 2 of the Iowa
Agricultural Experiment Station.)
}'mlll l'iUIIIIIy. Bacteria require food for growth just as truly as
do crops, all(! It is because of this nee:! that they influence fertility.
In the 11rocess of taldng IIJl food from the chemical cOUI!JOUnds in the
soil, the ha<;terla cause changes in the compound~. making them soluble and hence available for the growth of plants. :\lost soli bacteria
live principally on organic matter, or humus, and the products of
their own acth·ity. Some few S]:ecies are known which live In the
absence of organic materials. t:~ually, howe\·er, soils without humus
are without bacteria. Increasing the humus content. therefore; may
be expectedto increase the bacterial life. That Is actually the case
up to a certain limit, which varies widely for different soils.
9
FlO. 8.-Nodulel on Soy Bean root1.
Beyond a certain point, however, tl:e amount of food ceases to
govern bacterial growth and a lack of moisture or the Jlresence of
acidity or sourn ess may otf~ e : the bene fits of a !;reltf'r food SUJ»PIY.
The minerals In the soil salution aJ ; o ha\·c some Influence on the
bact e rl:~.
Certain groups are fav ored hy some sub!'tances and others
restricted or killc rl by certain othe~ caemicals. Thus the harterlal
floras or tht> !'oils of wet anrl dr~· regions ar" qalt e dlffPrent.
The bacteria furthermore not only act on the humus or organic
matter In the soli and bring about lt R solution In thf! 1•rocess of
obtaining their food, but they aim attack the min eral portion of the
soli and change Insoluble portions of that Into soluble.
B..ICTF:RIA ..IN{) THE TRA.\'.\'FOR.li..ITION OF
PLAXT FOO{)
We have con ~ hl e red the fact that the fertility of a ~oil i;. determined very largely by the bacterial activities goln~ on therein. \Ye
ha,·e dlscus!'ed briefly the nature of bacteria, their form, sl1.c, multi·
plication, etc., the numbers present In the soil and the effect of
various phy!llcal and chemical conditions In the soil on their development. We ha,·c found alfo that In the cours e of their life .1cth·ltles,
bacteria attack tl:e organic and mineral portion~ of the soli and
transform lnwlu!Jle cr. nstl:uent s Into forn15 foluhle anrl amllahl e for
crop nouri shment. It now ren~ain~ to s how how t:11s change Is ac-
10
FlO. 7.-Nodulea on Red Clover roots.
compllshed, to consider the various stages through which the transformation proceeds, to discuss the bacteria lnvol\"ed, and to reach
some conclusion regarding the Importance from the fertility standpoint of the changes brought about In this way In the store of plant
food constituents.
THE NITROGEN
PROBLE~I
AND ITS SOLUTION.
Soils are very apt to be deficient in nitrogen. This element, then,
Is generally the limiting factor in the growth of crops. Formerly
the lack of nitrogen In a soil was supplied by application of nitrate
o! soda, which was obtained from the nitrate beds In Chill. With the
Increasing demands for nitrates, the amounts taken yearly from the
nitrate beds were enormous and It was estimated that in a very short
time the deposits would be exhausted and t11e world would face a nitrogen famine. Of course, other nitrogenous materials were available for
manure, but In such smaiJ amounts that they would be merely a
drop In the bucket In supplying the demands of the world.
It was just at this crucial th1e that soil bacteriologists came to
the rescue and quieted the general fears by showing that certain
species of bacteria ll.\·ing In soli~ have the ability to draw upon the
Inexhaustible surJply ~f nitrogen In the air (which contains 79%
nitrogen) and fix It In the soil In a form available for plants. Thus
the nitrogen problem was solved and there need be no fear of a nitrogen famine.
:'\ITROGE:'\ FIXATIOX.
There are two classes of bacteria which are thus able to utilize
11
the nitrogen of the air in their growth: First, those which live entirely dependent on their own resources, and second, those which
grow on the roots of legumes such as clover, alralra, etc. The flret
are said to live non-srmbloUcall;r, or Independently, and are known
as non-symbiotic; the second are said to Uve In s:rmblosls with the
legumes, or In a state of mutual helpfulness and are called symbiotic.
NON-SY:\IBIOTIC NITROGEN FIXATION.
The first group of nitrogen fixers, or azotobacter as they are called
Is present in most soils. These bacteria rlx nitrogen In amounts
which haYe been estimated at from 15 to 40 lbs. per acre per :year
under ordinary conditions. One Instance Is on record where fixation
took place to the extent of 100 tons per acre to a depth of a few
Inches. This of course was excessive and all vegetation was killed.
This single Instance of "too much of a good thing" Is not to be
taken however as an argument against encouraging the actlvltles of
the azotobacter. Proper farm management Includes many practices
which encourage the fixation of nitrogen from the atmosphere. Thus
the ordinary operations of tillage, which open up the soil and admit
of a free circulation of air, encourage the growth of these free-living
bacteria and bring about greater fixation of nitrogen from the atmosphere. So also liming as a remedy for acidity increases the amount
of nitrogen In the soli by causing a greater growth of azotobacter.
In other words, any practice "'bleb brings about better physical and
chemical conditions In the soil for the growth of bacteria Increases
the activities of this particular group or organisms and the soil gains
In nitrogen content.
SYliBIOTlC
~ITROGE~ FIXATJO~.
The beneficial effect of clover growing on soils was known many
years before It was satisfactorily explained. The mystery was not
cleared up until the bacteriologists found that certain bacteria were
a!'sociated with legumes and that theFe bacteria took nitrogen from
the air and flxt>d It In the soli.
Legumes will ~rO\\' and flourish on soils that have absolutely no
nitrogen If the proper bacteria are present and the legn:nes become
lnorulntt'll with these bacteria. ln this process of Inoculation the
bacteria enter the roots of the legumes. The plants aid the process
by a softenlnJt of their tissue and then In so-calletl "Infection-threads''
the bacteria pas;; from cell to cell. They gather at a particular spot
and, nourished h)' the plant, multlpl)' to a large extent and form what
are known as nmlniP~, or swellings on the roots. As 11oon a~ ·the
organisms begin to multiJliY they begin to tak!' nitrogen from the
atmos)lhere and to supply It to the planL The plant In return SliP·
plies the bacteria with the necessary carbonaceous food and a close
union for mutual benefit Is thereby establlshe:l. So the legumes draw
but a small proportion of their nitrogen from the soli and H tlle
12
entire crop Is turned under for a ·g reen manure, · which Is a common
practtce, there Is a· large gain to tire soil In nitrogen.
1 . Legu\ues will often grow without Inoculation and In soils very
rlclr· hi nitrogen wlli yield good crops. They then draw their entire
nourishment from the soil. When that is the case the legumes ha\·e
no · advantage ,over the non-leguminous crop. But when legumes are
Inoculated they contain .a larger percentage of nitrogen and the soil
Is not robbej of Its stoclc of nitrogen.
SOIL INOCULATION.
The bacteria necessary for the Inoculation of legumes are not
always present In soils and hence we have to resort to what is known
as soil inoculntlon, which consists in Introducing the proper organisms
into the soil. This may be done In two ways: By Inoculating the
seed with pure cultures, that Is, cultures of the particular organism
grown on some convenient medium, or by S(Jreadlng on the land to
be seeded, soil from a field where the Jlarticu\ar legume to be grown
has been successfully grown previously.
Several commercial concerns, the United States department of
agriculture. and some experiment stations pretJarc cultures for the
inoculation of legumes !Jut none has as yet proved to be absolutely
satlsfactory. Cultures arc not always certain of accomplishing Inoculation and consequently we still recomn:end the use of soli for
FlO. 8.-Nodulea on Crimson Clover roots.
13
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A
FlO. 9.-Stalned preparations or Bacillus Mycoldea (A) and Baclllu' Subtllle
(B) with eporea, two prominent aoll ammonlflera.
the purpose. Three hundred to 500 pounds or soli per acre should
be uniformly distributed, just previous to see:llng o\·er the field to
be Inoculated, and dlsked ln. In this way bacteria In an acth·e state
or multiplication are Introduced without material change In the
character or their surroundings and they become vlgoroul! and ca!IIIY
enter the roJts of the Je,;ume, Insuring Inoculation and a good crop.
A~DIO:o;JFICATIO:o;.
The actl\·ltles of soli bactcrla with regard to the nitrogen problem arc l:n11ortan: not cnly from the standpoint of Increasing the
nitrogen content of soils thrJugh additions from the atmosphere but
also In the c~ange of organic materials Into available forms
Plant nn :l animal remains in the !'Oil, farmyard manure, green
manures. or other organic materials added to the !loll contain Insoluble organic nitrogenous matter known as Jlrnlt'ln and the.; e must
be chlnged lnt:J soluble nllralt'!' to be of U5e to plant~. This Rolutlon
Is acco:nplished by the process of •l«"rnr. Bacteria are the actl\'e
agents bringing about this decay In the soil. Varlou!l groups of universally dl5trlbuted organiRms are Involve:!. In the first Jllace the
lnsolttble 11r:JtelnR are changed Into wlublc Jlf'Jlhlht'!', t!lt>!le ar~
changed to nmlno nd•l~ and these In turn to nmmnnln. This change of
protein Into ammnnia IR calle :l nmmonlflrntlon, and It con~<tltntes a
vital ~tage In the production of nltrateg In the ROll. It Is Important
also In that It brings about the formation an1l later the destruction
or hmtttt!i,
Humus Is 1iecaylng organic matter In the soil. We know that
the JlfCt!ence of a proper amount of humus ·and also the best r;. •'
o{ destruction are Important factors from the physical and chemi ~ :JI
standpoint In determining the fertility of a soli. The ammonlfylnt;
bacteria attack the organic materials In the soli and bring about t:1e
14
formation of humus. This action continues and If no more organic
substances are supplied, the humus in the soil Is used up. Furthermore, If the soil is too open, too much air is admitted and the bacterial activities are so strong that the humus Is destroyed and fertility
is lost. Thus in sandy soils it Is very difficult to keep up the humus
content and recourse is had to green manuring for upbuilding purposes. In ordinary soils, however, the destruction of the humus is
not so rapid, but In any case if the soil does not receive additions
of organic matter at more or less frequent Intervals, it will become
deficient In humus.
Introduction of barnyard manure brings about vastly increased
bacterial action due to the large amount of organic matter added and
also the large number of bacteria Introduced. Thus the ammonifylng power of a soil is Increased by addition of manure. Liming
also has been shown to increase the activities of the ammonifying
bacteria and coasequently to hasten the production of plant food. All
tillage which admits air to the soil Increases the ammonia producing
power of the soil and later the nitrate production
NITRIFICATIOX.
Ammonium compounds produced In the soil as just described
never accumulate to any apprecla"llle extent but are transformed Into
nitrates almost as rapidly as they are formed. This transformation
Is called nltrJflentlon and Includes two stages: the change of ammonia to nitrites and then the oxidation of nitrites to nitrate~. Two
distinct classes of organisms are Involved in the process and they
are both of practically unh·ersal distribution.
All the farming operations which Increase anunonification havtl
a slmllar effect on nitrification, since nitrification starts where ammonification leaves off.
Particularly necessary for nitrification,
however, is the presence of lime In soils. This is due to the fact that
nitrous and nitric acids are produced in the process of nitrification
and If they are not neutralized by lime they accumulate and very
quickly stop bacterial action.
The nitrifying bacteria are al~o senslth·e to an excess of organic
matter but the amounts present in ordinary soils are never large
enough to. depress nitrification.
It Is evident then that the activities of both the ammonlfying
and the nitrifying bacteria arc governed ,·ery closely by the climatic
and farming conditions with regard to moisture, temperature, acidity,
aeration, and food supply,
DENITRIFICATION.
There is one process which is the result of bacterial action and
is deser\·ing of mention here, if for no other reason than to set aside
suspicions as to its common occurrence in the soil. It is denltrlil·
cation. Denitrifi~atlon Is a process brought about by certain bacteria
leading to the loss of gaseous nitrogen Into the atmosphere. If this
loss In the field were considerable the Importance of the process
would be evident, As a matter of fact, however, the danger of loss Is
very remote. The conditions necessary for denitrification, which are
excessive organic matter, large amounts or water, nitrates and the
proper bacteria, are rarely met with In the soli. Only when exceedIngly large amounts of fresh barnyard manure and sodium nitrate
are applied together, a practice which Is commonly condemned,
could denltriflc:•tlon occur. DetJresslon In crop yields which have
sometimes been observed when large quantities of manure ha\·e been
applied and which have been attributed to denitrification were more
likely due to some other cause, perha11s to the soluble organic
matter lnt: ~duced. With ordinary aptlllcatlons of barnyard manure,
the farmer need have no fear of losing nitrogen from his soil bJ
this process.
BACTERIA AND MINERALS IN THE SOIL.
In the process of decay of which we ha\·e spoken, the destruction
of the organic nitrogenous materials leads to the production of other
compounds than ammonia and nitrates. Chief among these Is carbon
dJoxlde. Furthermore, the organic non-nitrogenous substances, such
as starches, sugars, cellulose, etc., are destroyed In the general decay which occurs In the organic matter and among the nrlety of
products which result we find l"arlous organic acids and particularlY.
carbon dioxide.. These organic acids and the carbon dioxide have
the power of attacking Insoluble mineral compounds In the soil and
transforming them Into soluble forms, available for plant food. Thus
Insoluble phosphates and potash compounds are acted upon and
changed into soluble forms by the soil water carrying the carbon
dioxide and organic acids In solution. Again we see that bacterial
activities bring about the preparation of plant food.
The demands of Yarlous crops for sulfur has been the subject
of recent ln\·estlgaUons and it has been estimated that the supply
of sulfates In the soil may be insufficient In many cases for the
proper feeding of certain crops. A group of organisms occurring in
the soli and known as the sulfur hncteria come Into prominence here
as the agency keeping up the proper supply of sulfates. When
proteins decay, hydrogen sulfide gas Is set free. This Is taken up
by the sulfur bacteria and oxidized to free sulfur which Is In turn
oxidized to sulfates. Increased decar therefore leads to Increased
hydrogen sulfide and this In turn to increased sulfates for plant food.
We may conclude, therefore, that all methods which Increase
the acth·ltlcs or the decay bacteria lead directly to an Increased
supply or a\·allablc nitrogen, and Indirectly to larger amounts or
phosphoru~. pota!!slum, and sntrnr becomln~t a\·allable for plant
food.
16
CONCLUSION.
In conclusion the fact must be e:nphasized that the bacterial
processes going on In the soil cannot be lgnore:l in a consideration
of Its fertility. The Jlhyslcal and chemical character of the soil alone
wJll not tell us Its crop prmluclng power and we must depend on
the results of tests of bacterial activities. The recent development
of methods In this direction gives ns rels:>n to hope that In the near
future bacl erial tests of fertility may become of considerable practical
\"alue.
From the Jlractlcal standpoint, It should be evident that the
greatest care ought to be exercised on every farm to maintain condltlons satisfactory for the best growth of beneficial bacterial
species. )lolsture conditions should be governed as far as possible
by drainage or cnltivatl::m, aeration should be carefully regulated
to keep the dc~trnctlon of the humus from procee:ling too rapidly;
and the reaction of the soli should not be allowej to beco:ne acid,
adding lime if necessary to 11revent It. If these c;:,ndltions are carefully governed and the humus content of t:-:e soil is properly maintained by manuring with farmyard or green manure, and ]Jroper
rotations containing a legume are employe:l, the bacteria can be de~
pended upon to perform their part faithfully an:! well an:l the crop
wlll not fail for lacl< of sufficient available nitrogm.
Furthermore, if c!lemlcal analyse;; have shown sufficient amoun-ui
of the necessary mineral plant food con5tltuents. the bacteria under
the O]Jtlmum conditions just outlined will transform It Into an available form to satisfy the needs of the growing crop. .In short, the relation between bacteria and soil fertlllty is very
close and very \"ltal and systems of permanent agriculture must rest
firmly on a bacteriological basis to be of any value.